Photopolymerizable compositions comprising a carboxyl group-containing, film-forming polymeric binder, an addition photopolymerizable monomer, a thermal polymerizable inhibitor and a free radical photoinitiator are known. These compositions, are called "dry films" and can be developed by aqueous media because of the presence of the carboxyl groups in the polymeric binder. They are often sold as a roll of dry film sandwiched between a flexible support and cover members.
Such photopolymerizable compositions are shipped as dry films and are often subject to cold flow, i.e. the composition flows under pressure, which leads to material fusing together at the edges of the roll (edge fusion). The degree to which this occurs appears to be related to the viscosity of the film. It is desirable to increase the viscosity of the film in order to prevent or at least reduce edge fusion.
Such photopolymerizable compositions are often used as photoresists in the manufacture of printed circuit boards and it is desirable that they be flexible. The more flexible the film is in the polymerized state, the less likely it is to crack and break away from the printed board during board flexing or line trimming.
It is also desirable for a polymerized (cured) film to be strong enough to protect through-holes on a circuit board from processing chemicals and plating. To do this, film must be able to span through-holes up to about 7.0 mm in diameter and remain intact during processing. Spanning of such holes is referred to as tenting.
It is further desirable for the polymerized film to have a level of adhesion to copper following lamination that the film will not lift off the copper during trimming, stacking or removal of cover member prior to development. The film should also maintain good contact with the copper during developing, plating and etching.
In the past, maintaining the flexibility of the cured film has been accomplished by the addition of plasticizers. However, it has been found that the addition of plasticizer has an adverse effect on tenting and increases the amount of cold flow, leading to edge fusion.
It is therefore an object of the present invention to provide a photopolymerization composition which can be used to form dry films having an advantageous combination of properties in the cured and uncured states.